Embodiments presented herein relate generally to monolithic structures, and more particularly to electrically resistive monolithic structures.
One common method of controlling or varying current or voltage in an electrical circuit is by the use of a variable-resistor or rheostat. Typically, for electrical equipment with a high power rating, rheostats may be used for equipment startup and shutdown. For instance, brushed DC motors may have a manual rheostat starter, a three point rheostat starter, a four point rheostat starter, or the like, to gradually increase armature current from a small value to the rated operational value to protect the DC motor. However, during operation the rheostat switches from one resistance branch to another in a step manner. Such switching operation may result in arcing between the moving contact and the resistance branches, due to the high voltage and/or current supplied to the electrical equipment.
Another known solution includes multiple discrete resistive layers assembled in a stack, with the moving contact designed to slide over the stack. However, with operational wear, the surface of the stack in contact with the moving contact (referred to herein as “sliding surface”) is ground down by the moving contact, thereby resulting in an uneven sliding surface. Therefore, the moving contact may not maintain adequate contact pressure over the length of the stack. Further, the different resistive layers of the stack may be ground down to different levels based on the hardness of the resistive layer, thereby forming ‘steps’ on the sliding surface. This may result in the possibility of the moving contact being stuck at a step between successive resistive elements. Uneven contact pressure and the stepped profile of the sliding surface may both lead to arcing between the moving contact and the sliding surface. Also, such assembled stacks are difficult to machine and polish to obtain a smooth sliding surface. Often, the assembled stacks crack or break during machining.
Therefore, there is a need in the art, for a resistive element that addresses these and other shortcomings associated with known solutions.